The invention relates to composite superconducting oxide articles.
Since their discovery, oxide superconductors based on copper oxides have been widely studied. A key property is that the materials exhibit superconductivity at high temperatures relative to their traditional metallic counterparts. In many applications, it is important for the superconducting oxides to be composed of substantially one phase and have critical current densities (Jc) that are high.
One family of oxide superconductors includes bismuth-strontium-calcium-copper-oxide compositions such as Bi2Sr2CaCu2O8-x, (BSCCO-2212; where x is a value that provides a Tc of about 80K) and Bi2Sr2Ca2Cu3O10-y (BSCCO-2223; where y is a value that provides a Tc of at least 100K). Of particular interest are compositions where bismuth is partially substituted by dopants such as lead (that is, (Bi,Pb)SCCO)
The oxide materials, being ceramics, are generally brittle and are difficult to process and manipulate. Composites of oxide superconducting materials contained in metal matrices, or metal sheaths, have mechanical and electrical properties that are improved relative to the oxide superconductors alone. The composites can be prepared in elongated forms such as wires and tapes by processes, such as the well-known powder-in-tube (PIT) process, that typically include a number of stages.
In the PIT process, first, a powder of a precursor to a superconductor is prepared. The precursor can be a single material or a mixture of materials. Second, a metal container (for example, a tube, billet or grooved sheet) is filled with the precursor powder. The metal container serves as a matrix, constraining the superconductor. Third, the filled container is deformed in one or more iterations (with optional intermediate annealing steps) to reduce the cross sectional area of the container in a draft reduction step. A number of filled containers (filaments) can be combined and surrounded by another metal matrix to form a multifilament article. Finally, the material is subjected to one or more deformation and phase conversion heat treatment cycles which together form the desired oxide superconductor from the precursor and helps the oxide superconductor grains align and grow to form the textured superconductor article.
If the precursor powder is composed of one or more oxides, the process is known more specifically as oxide-powder-in-tube (OPIT) processing. See, for example, Rosner, et al., xe2x80x9cStatus of superconducting superconductors: Progress in improving transport critical current densities in superconducting Bi-2223 tapes and coilsxe2x80x9d (presented at the conference Critical Currents in High Tc Superconductors"", Vienna, Austria, April, 1992), and Sandhage, et al., xe2x80x9cThe oxide-powder-in-tube method for producing high current density BSCCO superconductorsxe2x80x9d, Journal of Metals, Vol. 43, No. 3, 1991, pp. 21-25, all of which are incorporated herein by reference.
A method of preparing BSCCO superconducting materials, particularly lead-doped BSCCO, is described in U.S. Ser. No. 08/467,033 filed Jun. 6, 1995 now U.S. pat. No. 5,942,466 and entitled xe2x80x9cProcessing of (Bi,Pb)SCCO Superconductor in Wires and Tapes,xe2x80x9d and U.S. Ser. No. 08/331,184 filed Oct. 28, 1994 and now U.S. Pat. No. 6,295,716 entitled xe2x80x9cProduction and Processing of (Bi,Pb)SCCO Superconductors,xe2x80x9d both of which are incorporated herein by reference. In particular, the composition of the precursor powder is controlled to improve processing of the precursor powder. One feature of the BSCCO-2223 processing path described in this patent application is that tetragonal BSCCO-2212 (T-2212) is thermally converted to orthorhombic BSCCO-2212 (O-2212) prior to formation of BSCCO-2223. The phases can be distinguished by their X-ray diffraction patterns and lattice parameters.
Texturing helps to increase Jc of the article. Texturing can be accomplished by deformation (deformation-induced texturing; DIT) or by phase conversion heat treatments which cause, for example, sintering or partial melting and regrowth of desired superconducting phases (reaction-induced texturing; RIT). Certain superconducting oxides, particularly BSCCO-2223 require precursor Deformation for adequate performance. Density and texture of the product is increased in the article by sequential repetition of the deformation (D) and phase conversion heat treatment, or sintering (S), steps xe2x80x9cnxe2x80x9d times (nDS). Typical processes are 1DS to 5DS. Multiple low reduction deformation steps are described, for example, in Hikata, et al., U.S. Pat. No. 5,246,917, incorporated herein by reference. A single high reduction deformation step which can be used in 1DS processes is described in U.S. Ser. No. 08/468,089 filed Jun. 6, 1995 now U.S. pat. No. 6,247,224 and entitled xe2x80x9cSimplified Deformation-Sintering Process for Oxide Superconducting Articlesxe2x80x9d, incorporated herein by reference.
In all stages of processing, it is important to avoid the formation of undesirable secondary phases. The presence of secondary phases can disrupt DIT of the primary phase of the precursor powder, can lead to gas evolution and can induce melting during heat treatments.
The metal container, sheath, or matrix which holds the precursor powder prior to and during processing is typically composed of a noble metal such as silver, gold, platinum, and palladium, or an alloy substantially comprised of a noble metal. A noble metal is a metal which is substantially unreactive with the oxide superconductor or the precursor powder under processing and use conditions. Oxide dispersion strengthened (ODS) silver alloys have been used as matrix materials. The silver alloy is exposed to an oxidizing atmosphere to generate the metal oxide dispersion in the matrix, thereby increasing the strength and hardness of the matrix. In general, when oxidation occurs through reaction with atmospheric oxygen, the outer portion of the article becomes harder than the center, decreasing the flexibility of the article. See, for example, Lay, U.S. Pat. No. 5,384,307, incorporated herein by reference. Lay co-oxidizes the matrix and the precursor during the step that forms the desired (final) oxide superconductor by heating the article in an oxidizing atmosphere of about 3 to 14 volume percent oxygen. One problem of this process is that the superconductor can be xe2x80x9cpoisonedxe2x80x9d as the matrix and precursor compete for oxygen. In practice, small amounts of poisoning can drastically reduce Jc. Thus, many ODS silver-superconductor composites have lowered current carrying capabilities. U.S. Ser. No. 08/626,130 filed Apr. 5, 1996 now U.S. Pat. No. 5,914,297 and entitled xe2x80x9cOxygen Dispersion Hardened Silver Sheathed Superconductor Compositesxe2x80x9d, incorporated herein by reference, describes one solution, but it requires metallic alloy precursors and cannot be practiced with oxide precursors. Since OPIT-based composites generally have good Jc performance, it is desirable to find a readily manufacturable, non-poisoning OPIT processing route to high performing ODS superconducting composites.
We have discovered high performing composite superconducting oxide articles that can be produced from OPIT precursors substantially without poisoning the superconductor. In general, the superconducting oxide is substantially surrounded by a matrix material. The matrix material contains a first constraining material including a noble metal and a second metal. The second metal is a relatively reducing metal which lowers the overall oxygen activity of the matrix material and the article at a precursor process point prior to oxidation of the second metal. Typically, the precursor article includes a precursor oxide mixture substantially surrounded by a first constraining material. In a preferred embodiment, a second constraining material substantially comprised of a noble metal or its alloy is included between the precursor oxide material and the first constraining material.
The processing sequence includes two or more phase conversion heat treatments by which the precursor oxide mixture is converted to the desired superconducting oxide, either directly or through one or more intermediate phase conversions. Between heat treatments, the article is deformed to texture the oxide mixture. The second metal is substantially converted to a metal oxide dispersed in the matrix during or prior to the first phase conversion heat treatment but after formation of the composite, creating an ODS matrix. Thereafter, the precursor oxide mixture is substantially converted to the desired superconducting oxide in one or more subsequent phase conversion heat treatments with intermediate deformation. In one embodiment, the precursor oxide mixture is partially reduced during the first of these heat treatments, in which the second metal can be oxidized.
The first heating step includes an oxidizing heat treatment step in which the second metal is substantially oxidized prior to or concurrent with a first phase conversion heat treatment step. The oxidizing heat treatment can be a separate step. In some embodiments, the oxidizing heat treatment is a stepped or ramped portion of the first phase conversion heat treatment. In still others, the oxidizing and first phase conversion steps occur concurrently.
In another aspect, the invention features a method of manufacturing a composite superconducting oxide article by providing a precursor article including a precursor oxide mixture containing a dominant amount of a first precursor oxide phase which is substantially surrounded by a first constraining material including a noble metal and a second metal, heating the precursor article in a first phase conversion heat treatment at a processing temperature and oxygen partial pressure cooperatively selected to convert up to about 85 percent, preferably up to about 50 percent, most preferably between about 20 and 40 percent, of the first precursor oxide phase to the desired superconducting oxide and substantially completely converts the second metal to a metal oxide, deforming the precursor article to generate texture, and, thereafter, heating the precursor article in one or more subsequent phase conversion heat treatments to convert substantially all of the remainder of the precursor to the desired superconducting oxide.
In another aspect, the invention features a method of manufacturing a composite superconducting oxide article by heating a precursor article in a first phase conversion heat treatment at a processing temperature and oxygen partial pressure cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase, deforming the precursor article to generate texture, and heating the textured precursor article in at least one subsequent phase conversion heat treatment to obtain the desired superconducting oxide. The article can benefit from redox processes between the matrix and a precursor oxide mixture in the first phase conversion heat treatment. The precursor oxide mixture contains a dominant amount of a tetragonal BSCCO phase and the first constraining material includes a noble metal and a reducing metal. The orthorhombic BSCCO phase can be formed in part by reducing the precursor oxide mixture with the reducing metal. In the process, the reducing metal can be oxidized by the precursor oxide mixture to form metal oxide particles dispersed in the first constraining material. In preferred embodiments, the method includes the step of roll working the precursor article in a high reduction draft of between about 40 to 95 percent prior to the sintering step.
In another aspect, the invention features a method of manufacturing a composite superconducting oxide article by providing a precursor article including a precursor oxide mixture substantially surrounded by a matrix including a first constraining material including a noble metal and a second metal, heating the precursor article to form less than about 0.5 volume percent of metal oxide particles of the second metal dispersed in the first constraining material, and sintering the heated article to obtain the superconducting oxide article.
In another aspect, the invention features a method of manufacturing a composite superconducting oxide article by providing a precursor article including a precursor oxide mixture substantially surrounded by a matrix including a first constraining material which includes a noble metal and a second metal, heating the precursor article to form less than about 0.3 volume percent of metal oxide particles of the second metal dispersed in the matrix, and sintering the heated article to obtain the superconducting oxide article.
In preferred embodiments, the article has a tensile strength greater than about 50 MPa.
In another aspect, the invention features a composite superconducting article which contains a plurality of filaments extending along the length of the article, where the filaments are composed, in part, of an oxide superconductor. A first constraining material substantially surrounds the plurality of filaments and a second constraining material substantially surrounds each of the filaments and is substantially surrounded by the first constraining material. The filaments can have substantially the same critical current density regardless of the location of each filament in the article. In preferred embodiments, the first constraining material includes a noble metal and a metal oxide and the second constraining material contains a noble metal. In other preferred embodiments, an outermost constraining material substantially surrounds the first constraining material, and contains a noble metal. In this embodiment, the outermost constraining material can include a noble metal, a noble metal and a reducing metal, or a noble metal and a metal oxide. In preferred embodiments, the outermost constraining material is silver.
In another aspect, the invention features a composite superconducting article including an oxide superconductor substantially surrounded by a first constraining material where the first constraining material includes a noble metal and less than about 0.5 volume percent of a metal oxide. In another aspect, the volume percent of the metal oxide is measured across the total cross-sectional area of the matrix by omitting the areas corresponding to the superconducting oxide from the cross section. In yet another preferred embodiment, the metal oxide particles dispersed in the constraining materials occupy less than 0.3 percent of the volume of the matrix. More preferably, the metal oxide particles occupy between 0.001 and 0.3 percent of the volume of the matrix, and most preferably at least 0.005. The matrix can have relatively metal oxide rich and metal oxide poor regions and these percentages are calculated as the average over all regions of the matrix.
In other aspects, the invention features a composite superconducting article including one or more filaments extending along the length of the article. The filaments include an oxide superconductor. A first constraining material substantially surrounds the plurality of filaments. A second constraining material substantially surrounds each of the filaments and is substantially surrounded by the first constraining material. The first constraining material includes a noble metal and less than about 0.5 volume percent of a metal oxide and the second constraining material includes a noble metal.
In preferred embodiments, the noble metal includes silver, gold, platinum, or palladium or their alloys and the reducing metal, metal of the metal oxide, or second metal is aluminum, magnesium, lithium, sodium, potassium, calcium, beryllium, strontium, barium, yttrium, scandium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, silicon, germanium, tin, lead, gallium, indium, thallium, manganese, antimony, or zinc. More preferably, the reducing metal, metal of the metal oxide, or second metal is aluminum, magnesium, yttrium, zirconium, titanium, hafnium, lithium, manganese, antimony, or zinc. In preferred embodiments, the reducing metal forms a solid solution with the noble metal.
The article can contain from 1 to 10,000 filaments. In preferred embodiments, the composite superconducting articles can have a plurality of oxide superconductor filaments. Most preferably, the multifilament article can contain from 5 to 1,000 filaments.
The invention can be practiced with any micaceous or semi-micaceous superconducting oxide. Grains of these materials tend to align during deformation and so are particularly well suited to texturing by xe2x80x9cDSxe2x80x9d processes. Members of the bismuth family of superconducting copper oxides are particularly preferred.
In certain embodiments, the tetragonal BSCCO phase is tetragonal BSCCO-2212, the orthorhombic BSCCO phase is orthorhombic BSCCO-2212 and the desired superconducting oxide is BSCCO-2223. Preferably, the tetragonal BSCCO phase is tetragonal (Bi,Pb)SCCO-2212, the orthorhombic BSCCO phase is orthorhombic (Bi,Pb)SCCO-2212 and the desired superconducting oxide is (Bi,Pb)SCCO-2223. The desired superconducting oxide can be orthorhombic BSCCO-2212 or orthorhombic (Bi,Pb)SCCO-2212.
The invention may provide one or more of the following advantages. The use of multiple phase conversions provides a better textured product. Intermediate deformations also improve texture, and the relatively low loading of oxide during deformation improves formability as well as reduces edge cracking. The presence of the second metal in the matrix can lower the oxygen activity within and throughout the material allowing better control of the oxidation reactions. Previous methods relied on the diffusion of oxygen from the precursor oxide mixture to the atmosphere. The second metal can provide a xe2x80x9csinkxe2x80x9d for the oxygen, eliminating the need for the oxygen to diffuse completely out of the article during processing. Relatively low loadings of reducing materials can help balance these reactions for controlled phase conversion. ODS-strengthened composites exhibit better mechanical properties. ODS formation prior to deformation also helps promote densification of the filaments, which mechanically accelerates reaction process because the increased filament density can shorten heat treatment times. The articles produced can also have increased critical current densities (Jc, or Je)
It is possible to use a complete reducing matrix material where the precursor oxide powder is in direct contact with unoxidized matrix alloys.
xe2x80x9cArticle,xe2x80x9d as used herein, means a wire, tape, current lead, or cable, and devices fabricated from these. The wire and tape can have more than one type of filament. The article can contain a single oxide superconductor filament (monofilament) or a plurality of oxide superconductor filaments (multifilament).
xe2x80x9cMatrixxe2x80x9d means the material surrounding the oxide superconductor filament or filaments in the article. The amount of matrix material can be measured by omitting the areas corresponding to the superconducting oxide filaments from the total cross section of the article.
xe2x80x9cPredominantlyxe2x80x9d or xe2x80x9cdominant amount,xe2x80x9d as used herein, mean at least 50 percent, preferably at least 90 percent and most preferably at least 95 percent of the desired phase is present by volume.
xe2x80x9cPhase modification,xe2x80x9d or xe2x80x9cphase conversion,xe2x80x9d means to alter the crystalline phase of the precursor oxide mixture from one form to another. For example, tetragonal BSCCO-2212 is phase modified to orthorhombic BSCCO-2212, or BSCCO-2212 is phase modified to BSCCO-2223.
xe2x80x9cPhase conversion heat treatment,xe2x80x9d as used herein, means a heat treatment by which some portion of the precursor oxides are phase converted into the desired oxide superconductor or a desirable intermediate phase. xe2x80x9cUndesirable secondary phase,xe2x80x9d as used herein, means a phase (or phases) in the precursor powder or superconducting oxide that hinders conversion of the precursor powder into the superconducting oxide, or hinders texturing of the superconducting oxide.
xe2x80x9cT to O conversionxe2x80x9d means the induction of a phase change from a tetragonal to an orthorhombic phase.
xe2x80x9cJc,xe2x80x9d the critical current density, means the current of the superconducting article normalized by the cross-sectional area of the oxide superconductor in the article. xe2x80x9cJe,xe2x80x9d the engineering current density, means the current of the superconducting article normalized by the total cross-sectional area of the article.
Other features and advantages of the invention will be apparent from the description of the preferred embodiment thereof, and from the claims.